Liquid crystal display devices are characterized by the high quality of images, the small thickness, the light weight and the small consumption of electric power and widely used for televisions, personal computers and automobile navigators. In a liquid crystal display device, two polarizers are disposed above and below a liquid crystal cell in a manner such that the transmission axes are perpendicular to each other. Images are exhibited on a display by changing orientation of liquid crystal molecules under application of a voltage to the liquid crystal cell. Many of the liquid crystal display devices of the twisted nematic mode have a construction such that the liquid crystal molecules are oriented perpendicularly under application of a voltage to exhibit a dark display. Many of the liquid crystal display devices of the in-plane switching mode have a construction such that the liquid crystal molecules are oriented in a specific direction in the absence of a voltage, and the direction of the orientation is rotated by 45 degrees under application of a voltage to exhibit a bright display.
In a liquid crystal display device in which the transmission axes of two polarizers are oriented in a manner such that one of the transmission axes is in the vertical direction, the other is in the horizontal direction, and the two axes are perpendicular to each other, a sufficient contrast can be obtained when the display screen is viewed in the horizontal and vertical directions. However, when the display screen is viewed in an oblique direction deviated from the horizontal and vertical directions, birefringence takes place in the transmitted light, and leak of light arises. Therefore, a sufficient black depth is not obtained, and the contrast decreases. To overcome this problem, prevention of the decrease in the contrast is attempted by adding a means for optical compensation to the liquid crystal display device.
For example, a liquid crystal device of the in-plane switching mode in which the first polarizing plate, a film for optical compensation, the first substrate, a liquid crystal layer, the second substrate and the second polarizing plate are disposed in this order, one of the polarizing plate has a transmission axis parallel to the slow axis of the liquid crystal during the dark display of the liquid crystal layer, the other polarizing plate has a transmission axis perpendicular to the slow axis of the liquid crystal during the dark display of the liquid crystal layer, and the angle between the film slow axis of the film for optical compensation and the transmission axis of one of the polarizing plates is 0 to 2 degrees or 88 to 90 degrees, is proposed (Patent Reference 1).
A liquid crystal display device which comprises a liquid crystal cell and polarizing plates and changes the direction of the major axis of the molecules of the liquid crystal within a plane parallel to the substrate by the change in the voltage applied to the liquid crystal cell, wherein a sheet for optical compensation is disposed between the liquid crystal cell and at least one of the polarizing plates, the sheet for optical compensation has the optically negative uniaxial property, and the optical axis is parallel to the face of the sheet, is proposed (Patent Reference 2).
However, none of these means are sufficient for obtaining a liquid crystal device exhibiting uniform and high contrast in viewing in any desired directions, and a further improvement has been desired.
[Patent Reference 1] Japanese Patent Application Laid-Open No. Heisei 11 (1999)-305217 (pages 2 and 3) (U.S. Pat. No. 6,285,430, B1)
[Patent Reference 2] Japanese Patent Application Laid-Open No. Heisei 10 (1998)-54982 (pages 2 and 3) (U.S. Pat. No. 6,184,957, B1)